The present invention relates to wire coating extrusion and, more particularly, an extrusion head for a core wire coated with a high viscous melt-coating composition, which is constructed by extruding the high viscous material such as piezoelectric material round the core wire.
A piezoelectric material is such a material that generates a voltage when a vibration is applied, and the piezoelectric ceramic and the polymer piezoelectric material are well known. The ceramic made of lead titanate zirconate, or the like is present as the former, and the monoaxial stretching polyvinylidene fluoride (PVDF), or the like is present as the latter. As the apparatus that utilizes this characteristic, the cord-like pressure-sensitive sensor is known. This cord-like pressure-sensitive sensor is a soft, long and narrow cord having a diameter of about 2.5 mm. This sensor is built around the power window of the car to prevent the accident, or built on the outer periphery of the automatic guided vehicle that runs about in the factory, the warehouse, or the hospital, otherwise this sensor is built on the fence, which is put around the site boundary, to sense the burglary. This sensor is convenient because such sensor can output a pressure change at the concerned portion as an electric signal, no matter which portion of this long and narrow cord is folded or no matter which portion of this cord is lightly touched by the foreign matter.
A structure of the cord-like pressure-sensitive sensor using the coated core wire as an object of the present invention is shown in FIG. 4.
In FIG. 4, 40 is a cord-like pressure-sensitive sensor. This sensor is constructed by coating a piezoelectric material 42 on a core wire (core electrode) 41 that extends in the axial direction, then winding a shield electrode 43 round the piezoelectric material 42, and then coating the outermost periphery with PVC (polyvinyl chloride resin) 44. The present invention is subjected to an extrusion head used to coat the piezoelectric material 42 round the core wire 41.
The cord-like pressure-sensitive sensor 40 employs a resin material having a heat resistance, which permits the use up to the operating temperature of 120° C., as a composite piezoelectric material. Thus, this sensor can be used in a higher temperature range (120° C. or less) than 90° C. that is the highest operating temperature of the polymer piezoelectric material (monoaxial stretching polyvinylidene fluoride) or the composite piezoelectric material (composite piezoelectric material consisting of chloroprene and piezoelectric ceramic powders), which are typical in the conventional art. The composite piezoelectric material consists of the resin having the flexibility and the piezoelectric ceramic, and also the flexible electrode consisting of the coil-like metal core electrode and the film-like outer electrode is employed. Thus, this sensor has the flexibility equivalent to the normal vinyl cord.
In addition, the cord-like pressure-sensitive sensor 40 has a high sensitivity equivalent to the polymer piezoelectric material, and particularly has the high sensitivity equivalent to the polymer piezoelectric material in a low frequency range (10 Hz or less) in which the catch of the human body is sensed. This is because the relative dielectric constant (about 55) of this composite piezoelectric material is larger than the polymer piezoelectric material (about 10) and thus reduction in the sensitivity in the low frequency range (10 Hz or less) is small.
The high heat-resistant composite piezoelectric material (piezoelectric material consisting of two different materials) is formed of the composite material that consists of the resin material and the piezoelectric ceramic powders whose size is 10 μm or less. The vibration sensing characteristic is implemented by the ceramic, and also the flexibility is implemented by the resin. Since noncrystalline polyethylene resin (molecular weight about 300,000) and noncrystalline polyethylene resin (molecular weight about 100,000) are compounded as the resin material, this composite piezoelectric material makes it possible to attain not only the high heat-resistance (120° C.) and the flexibility, which brings about the easy formation, but also simple manufacturing steps in which the bridging is not required.
The cord-like pressure-sensitive sensor 40 obtained in this manner does not have piezoelectric performance just after the piezoelectric material is molded. Therefore, the process of providing the piezoelectric performance to the piezoelectric material (polarizing process) must be executed by applying a DC high voltage of several kV/mm to the piezoelectric material. This polarizing process is executed by applying the DC high voltage to both electrodes after electrodes are formed on both sides of the composite piezoelectric material. If a minute defect such as a crack, or the like is present in the composite piezoelectric material, the short-circuit between both electrodes is ready to occur due to the discharge caused at the defect portion, so that the sufficient polarization voltage cannot be applied to the composite piezoelectric material. In this case, in the present invention, since the unique polarizing step employing an auxiliary electrode that can tightly contact to the composite piezoelectric material up to a predetermined length is established, the defect can be sensed/avoided to stabilize the polarization, whereby the longer cord having a length of several tens meter can be attained.
Also, in the cable-like sensor, the coil-like metal core electrode is employed as the inner electrode and the film-like electrode (triple-layered laminate film consisting of aluminum-polyethylene terephthalate-aluminum) is employed as the outer electrode. Thus, not only the adhesion between the composite piezoelectric material and the electrodes is assured but also the outer lead wire is easily connected to the sensor, so that the flexible cable-like fitting structure can be obtained.
The core electrode is made of the copper-silver alloy coil, the outer electrode is made of the triple-layered laminate film consisting of aluminum-polyethylene terephthalate-aluminum, the piezoelectric material is made of the polyethylene resin +the piezoelectric ceramic powders, and the housing is made of the thermoplastic plastic. Thus, the relative dielectric constant is 55, an amount of generated charge is 10-13 C(coulomb)/gf, and the maximum operating temperature is 120° C.
FIGS. 5A and 5B show an extrusion head of this type in the conventional art, wherein FIG. 5A is a longitudinal sectional view and FIG. 5B is a side view. In FIGS. 5A and 5B, 50 is an extrusion head, 51 is a melt-coating composition pressure-feeding portion, 511 is a land portion, 512 is a cap nut, 52 is a die ring, 521 is a die holding ring, 522 is a thickness deviation adjusting bolt, 523 is a tap bolt, 53 is a nipple, 54 is a die block, 55 is a mandrel, 56 is a nipple holder, 561 is a wire guide, 562 is a thickness deviation adjusting ring, 563 is a gap adjusting nut, and 57 is a composition temperature sensor.
Next, an operation of this apparatus in the conventional art will be explained hereunder.
First, the core wire W is guided linearly from the wire guide 561 on the left side in FIG. 5A, then is moved linearly through passages in the mandrel 55 and the nipple 53 from there to the right side in FIG. 5A, and then is pulled out from an exit portion of the die ring 52.
Meanwhile, the composite piezoelectric material consisting of the piezoelectric ceramic powders and the synthetic rubber is supplied from the melt-coating composition pressure-feeding portion 51 shown at the top in FIG. 5A and is heated by a heating apparatus (not shown). Then, such material is fed through a passage of the land portion 511 and a passage (not shown) of the mandrel 55 and a clearance 541, which is formed between an inner surface of the die block 54 and outer surfaces of the nipple 53 and the nipple holder 56, by a pressure-feeding mechanism (not shown) such as a screw, or the like in the melted state. Then, such material is output from an exit portion of the die ring 52 while coating round the outer side of the core wire W at the front end of the nipple 53.
As can be understood from a sectional view of the die ring 52, the internal shape of the die ring 52 in the apparatus in the conventional art is shaped into a taper (funnel) shape that an inner diameter is narrowed with the progress in the proceeding direction to prevent the stagnation of the melt-coating composition. It is considered that, because of this funnel shape, the melt-coating composition can be pressure-fed smoothly without the stagnation in the hollow portion. Therefore, this funnel shape is assumed as the best shape up to now and thus other shapes except this shape are not suitable for the pressure-feeding of high viscous composition. In other words, everybody thinks of such a conclusion that, if this melt-coating composition is formed of not the high viscous composition but the low viscous composition or the fluid liquid, the internal shape of the die ring should not always be shaped into the taper shape and thus such melt-coating composition can be pressure-fed smoothly even when the internal shape of the die ring is shaped into the perpendicular wall, e.g., a syringe, to the proceeding direction. However, this taper shape is an indispensable major premise in the extruding apparatus that extrudes the high viscous (so-called hard) composition such as the material of the pressure-sensitive sensor, and this shape is a ultimate shape. The existing state is that other shapes except the taper shape are not taken into consideration at all in this technical field.
Then, it is considered that it takes a lot of time to pressure-feed the melt-coating composition since such composition is high viscous, nevertheless the coated core wire can be manufactured at a production speed of 1 m/min because the internal shape of the die ring is shaped into the taper shape.
By the way, the applicant of this application tried various experiments to increase the above production speed of 1 m/min.
Experiment 1: First, in the apparatus in the conventional art, when the number of revolution of the screw was increased up to three times or more (7 rpm) of the normal number of revolution (2 rpm), the melt-coating composition was not ejected from the exit portion of the die ring 52 but such composition was leaked from the flange.
Experiment 2: Then, in the apparatus in the conventional art, when the flange was clamped once again after the temperature rise and then the number of revolution of the screw was further increased up to two times or more (14 rpm), similarly the composition was leaked from the flange. The extrusion speed was 1.5 m/min. Then, if the number of revolution of the screw was further increased, the bolts of the flange were broken.
Experiment 3: In the apparatus in the conventional art, when the flange was clamped once again while using the convexly projected copper packing after the temperature rise and then the number of revolution of the screw was set to 2 rpm, the melt-coating composition was not ejected from the exit portion of the die ring 52 but such composition was leaked from the flange. The pressure sensor sensed about 60 MPa.
Experiment 4: In the apparatus in the conventional art, the number of revolution of the screw was set to 2 rpm while using the existing packing and the Teflon packing, the melt-coating composition was not ejected from the exit portion of the die ring 52 but such composition was leaked from the flange. The Teflon packing was also protruded. The pressure sensor sensed about 50 MPa.
Experiment 5: In the apparatus in the conventional art, when the clamping of the bolt is increased while using the convexly projected copper packing and the Teflon packing and the number of revolution of the screw was set to 4 rpm, the extrusion speed was 0.6 m/min and the melt-coating composition was not ejected from the exit portion of the die ring 52 in the middle. The pressure sensor sensed about 95 MPa. Then, if the number of revolution of the screw was further increased, the bolts were broken.
As described above, the production speed of 1 m/min is the upper limit, and leakage of the composition from the packing or breakage of the bolt is caused if the production speed is increased further more.